Catheters of the type which are inserted into a vessel of a patient for carrying electrical signals to and from the patient are used in various applications. For example, cardiac catheters are inserted within a blood vessel into a patient's heart to detect cardiac electrical signals, to apply electrical stimulation for diagnostic testing and to apply treatment signals, such as tissue ablation signals which are used to eliminate the source of an arrhythmia. Other applications for ablation catheters include the treatment of tumors, such as breast or liver tumors, and the identification of tumor biopsy sampling sites. In addition to one or more electrodes, the catheter may include other structures, such as a lumen through which light, thermal energy or chemical agents are delivered and/or a sampling system for sampling a tissue or fluid specimen.
One multi-electrode catheter arrangement, described in U.S. Pat. No. 5,341,807 (Nardella), includes signal processing circuitry for detecting contact of the catheter with tissue, such as a vessel wall. The Nardella catheter includes a tip electrode and a plurality of ring electrodes spaced along the catheter. The differential voltage indicative of impedance between the electrodes is measured to provide an indication of the catheter electrodes being disposed in different mediums (for example, when one electrode is in blood and another is in contact with tissue). The resulting indication of catheter contact is useful in many applications. For example, in cardiac ablation, the catheter must be in contact with, or at least in close proximity to, the treatment site in order to ensure that an effective level of RF energy reaches the tissue.
It is generally necessary to utilize a visualization technique of some sort in order to guide the catheter to a desired site of diagnosis and/or treatment and to ensure that the catheter remains at the desired location. Additionally, it is often desirable or necessary to re-position the catheter at a particular location. For example, in applications in which a cardiac ablation catheter is used for diagnosis and subsequent treatment of an arrhythmia, the catheter is moved around the heart while cardiac electrical signals are monitored, following which one or more sites identified as being the source of an arrhythmia are ablated. Thus, during such a procedure, it is necessary to determine the location of the catheter as the electrical signals are monitored in order to facilitate re-positioning the catheter at the site of an arrhythmia for ablation. Further, during any catheter procedure, the catheter may slip and require re-positioning in order to successfully complete the procedure.
Catheter positioning and re-positioning has conventionally been achieved with the use of fluoroscopic techniques. However, since fluoroscopy typically provides only two-dimensional information, its accuracy in catheter positioning is limited. Furthermore, due to the potential risks associated with exposure to electromagnetic radiation, it is advantageous to limit the use of fluoroscopy.